Molecular and cellular regulation of mammalian renal edulla development The renal medulla plays critical roles in body water and salt homeostasis and the regulation of systemic blood pressure. Disorders in water and salt balance cause hyponatremia, hypernatremia and nephrogenic diabetes insipidus. Renal medulla atrophy is a prominent symptom of hydronephrosis, a common condition occurs in approximately 1 in 100 people and a consequence of acute and chronic urinary tract obstructions and neurological deficits. Elucidation of the molecular mechanisms required for renal medulla formation is important not only for the understanding of normal kidney organogenesis and physiology, but will also guide efforts in the development of new strategies of the repair and regeneration of the renal medulla. In spite of the importance of the renal medulla for kidney functions, little is known of the cellular and molecular mechanisms required for its formation. We recently showed that Wn7b signaling is essential for renal medulla formation, mostly likely through its regulation of the orientation of division of prospective medullary collecting duct cells. We showed that Wnt7b is required for canonical Wnt signaling in the renal interstitium and canonical signaling in the renal interstitium plays essential roles for renal medulla formation. How does Wnt7b and renal interstitial canonical Wnt signaling regulate oriented cell division of the prospective medullary collecting ducts and renal medulla formation? We found that both Wnt7b and renal interstitial canonical Wnt signaling are necessary for the renal interstitial expression of p57Kip2, a Cip/Kip family cyclin-dependent kinase inhibitor (CKI) that functions in both CKI-dependent and CKI-independent fashions. Interestingly, p57Kip2 is implicated in both sporadic and congenital Beckwith-Wiedemann Syndrome, a salient feature of which is renal medullary dysplasia, and ablation of p57Kip2 in mice dramatically reduces renal medulla elongation. These data suggest that p57Kip2 acts downstream of Wnt7b and renal interstitial canonical Wnt signaling in the regulation of renal medulla formation. We will test the hypothesis in this proposal (Aims 1 and 2). Oriented cell division in the epithelium is a planar cell polarity (PCP) phenomenon, in which the Wnt/PCP pathway has been implicated. Aim 3 will address the role of the Wnt/PCP pathway in mediating Wnt7b functions in renal medulla formation. Together these studies will significantly expand our knowledge on the establishment of the cortico-medullary axis of the kidney and the development and repair and regeneration of the renal medulla compartment, which is pivotal for physiology of the kidney and beyond. They will also provide mechanistic understanding of the pathogenesis of the renal medullary defects in diseases such as Beckwith-Wiedemann Syndrome.